Machines including a load handling apparatus typically include a front and a rear axle supporting a machine body on which the load handling apparatus is mounted. Wheels are normally coupled to the front and rear axles, the wheels being configured to engage the ground and permit movement of the machine across the ground.
The load handling apparatus includes, for example, an extendable lifting arm moveable by one or more actuators with respect to the machine body. The lifting arm includes a load carrying implement to carry a load such that a load carried by the load carrying implement can be moved with respect to the machine body by the lifting arm.
Movement of the load produces a moment of tilt about an axis of rotation of one of the front or rear axles. Alternatively, a moment of tilt may be induced about another axis where, for example, stabilisers are used to stabilise the body relative to the ground during load handling operations.
Extension of the lifting arm in forwards direction, particularly when carrying a load, induces a moment of tilt about the axis of rotation of the front axle. As a result the portion of the machine (and load) weight supported by the rear axle decreases.
In order to ensure that the machine does not rotate about the front axle to such an extent that the wheels coupled to the rear axle are lifted from the ground surface (i.e. to ensure that the machine does not tip), when the load on the rear axle reduces to a threshold level, a safety control prevents further movement of the lifting arm. An example of such a machine can be found in EP1532065.
A problem arises because, in order to remain within safety limits, the threshold level which is selected for use by the safety control is overly restrictive for certain lifting arm positions—preventing the lifting arm from being moved into positions which do not actually risk the tipping of the machine.
It will be appreciated that this and similar problems apply to other machines too.